1. Field of the Invention
The present invention relates to mixer circuits and particularly to mixer circuits for processing television/video signals.
2. Description of the Prior Art
FIG. 5 shows a push-pull mixing stage with bipolar transistors, such as described in “Taschenbuch der Hochfrequenztechnik”, Meinke, Gundlach, Springer-Verlag, 5th edition, 1992, page Q22. The active mixing stage includes a first bipolar transistor T11 500, a second bipolar transistor T12 502, a third bipolar transistor T21 504, and a fourth bipolar transistor T22 506. The bipolar transistors are respectively arranged as two transistor circuits such that the two transistors T11, T12 are connected to each other in the first transistor circuit so that their emitters are coupled, wherein the coupled emitters of the two transistors T11, T12 represent the RF input. The transistor circuit formed by the two transistors T11, T12 is controllable by two control inputs, i.e. the base terminals of the two transistors, wherein a local oscillator signal may be applied to the base terminal of the first transistor T11, and wherein an inverted local oscillator signal may be applied to the base terminal of the second transistor T12, i.e. the second control input. Analogously, the positive local oscillator signal LO and the negative local oscillator signal LO_ are coupled to the two transistors T21, T22 of the second transistor stage so that the negated local oscillator signal is applied to the base terminal of the transistor T21 of the right-hand transistor circuit, and wherein the local oscillator signal is applied in non-inverted state to the second control input of the second transistor stage, i.e. to the second transistor T22. Thus, also the second transistor circuit formed by the two transistors T21, T22 has an RF input, which is again at the coupled emitters, in addition to the two control inputs, i.e. the base terminals of the transistors 504 and 506.
Both transistor circuits have two IF outputs. Thus, the collector terminal of the first transistor, i.e. the transistor also providing the first control input, represents the IF output (in non-inverted state), while the collector terminal of the second transistor represents a second IF output at which the IF signal may be taken in inverted state. Analogously, the second transistor circuit is formed by the transistors 504 and 506. The left-hand transistor T21 represents the IF output, while the right-hand transistor T22 represents the inverted IF output.
In addition to the two transistor circuits described above, the known mixer shown in FIG. 5 also includes an RF supply stage formed by the two transistors T1 and T2. The positive RF signal may be supplied to a control input of the first transistor T1, while the inverted RF signal may be supplied to the control input of the second transistor T2. Further, there is a current source means Idc.
The mixing stage illustrated in FIG. 5 is thus based on two push-pull mixing stages, each formed with two npn transistors. The drive thus takes place by the RF supply stage. In other words, the transistor T1 controlled by the RF signal and the direct current source Idc may together be regarded as controlled current source providing both the RF signal and the supply current. The local oscillator signal required for frequency conversion, as mentioned above, is supplied to the base terminals of the transistors and selected so large that the transistors operate in switching mode.
The circuit shown in FIG. 5 is particularly well suited for monolithic integration. Input signal RF and oscillator signal LO are each supplied in push-pull, and the intermediate frequency is taken in a balanced way. Thus a reduction of the even order intermodulation products and a suppression of the oscillator signal are achieved. Good balancing is achieved in active mixing stages when the transistor parameters of the transistors used in the left-hand and right-hand branches are as equal as possible.
In the mixer circuit shown in FIG. 5, the two transistors T11 and T12 are actually switched back and forth with the local oscillator signal. This maintains the non-linearity resulting in the frequency conversion. Particularly for applications at higher frequencies or also for applications expecting high linearity, a problem is that the transistors do not switch synchronously with each other due to real parameter variations, etc., wherein the problem is aggravated when going to higher frequencies. However, due to the controllability of the circuit design, it is preferred, on the other hand, to use transistor mixers instead of diode mixers or varactor mixers.
Furthermore, the intermodulation strength of mixers and, particularly, of balanced and double-balanced mixers suffers particularly when the transistor parameters of the two transistors in a transistor circuit, and, in the case of the double-balance mixers, the transistor parameters of the transistors in the two amplifier circuits differ, which, in turn, becomes more and more noticeable when going to higher frequencies.
A lower intermodulation strength, in other word, an increase in energy in unwanted intermodulation products, results in reduced linearity and an unwanted interference of the IF signal.